BGE: Import Error No Module Named "Tkinter"
Tkinter will freeze your game and create a popup window, which is probably not what you want. You could install it, but I wouldn't suggest it.

I've found that making simple interfaces is harder in code (eg tkinter) than it is in BGE with objects. Of course, by the time you want combo boxes, you probably want a GUI library. And these libraries do exist. 
There is a nice one called BGUI you should look into. This writes into openGL ontop of BGE, so it won't freeze your game or create external windows.